Magnetically separable mesoporous silica microspheres with open pore systems have been synthesized in supercritical carbon dioxide (SC-CO2). In the experiments, using the excellent transport properties of supercritical carbon dioxide, an acetone solution containing iron(III) acetylacetonate [Fe(acac)(3)] was easily infiltrated into the mesopores of the microspheres. After thermal decomposition, the Fe(acac)3 was easily transformed into Fe3O4 nanoparticles. A comparison experiment verified that the carrying effect of Fe(acac)(3) using SC-CO2 was better than that observed for the common method. The magnetic microspheres were characterized by transmission electron microscopy, energy-dispersive X-ray analysis (EDAX), powder X-ray diffraction, and nitrogen adsorption-desorption. The magnetically separable property of the microspheres was also tested.